1. Field of the Invention
This invention relates to a shift control system for a change gear transmission and in particular relates to a shift control system for a change gear transmission permitting preselection of a shift or gear change which shift or gear change will be executed upon achievement of enabling conditions, such as a torque break permitting disengagement of the previously engaged gear and achieving synchronization for substantial synchronization of the clutch members of the selected gear. More particularly, the present invention relates to a shift control system of the preselection type including a prohibit means prohibiting execution of the preselected gear change until the operator signals the prohibit means, by acutation of a prohibit release means, that execution of the preselected gear change is presently desired.
2. Description of the Prior Art
Compound transmissions of the splitter or range type, or a combination thereof, are well known in the prior art as may be seen by reference to U.S. Pat. Nos. 3,105,395; 3,648,546; 3,799,002; 2,932,988 and 3,229,551, all of which are hereby incorporated by reference. The main section of such transmissions are typically controlled by the operator by means of a shift lever operating directly upon shift rails or by electric and/or pneumatic means, and the auxiliary section is typically shifted by means of a control lever or button located on or ad3acent the shift lever for operating the remote auxiliary shift mechanisms either pneumatically, hydraulically and/or electrically. Examples of such shift control systems for compound transmissions are well known in the prior art and examples thereof may be appreciated in greater detail by reference to U.S. Pat. Nos. 2,637,222; 2,763,290; 3,648,546; 3,799,002; 3,921,469; 3,983,979 and 4,194,410, the disclosures of which are all hereby incorporated by reference.
Blocked change gear transmissions of both the single and compound types are also well known in the prior art as may be seen by reference to above mentioned U.S. Pat. Nos. 3,799,002; 3,921,469; 3,983,979 and 4,194,410. Blocked transmissions of this type typically utilize jaw clutch assemblies which may be axially shifted against a resilient bias for positive clutch engagement upon the sensing of a synchronous or substantially synchronous rotation therebetween. The structure and function of the resiliently biasable jaw clutch assemblies and of the blockers carried thereby are well known in the prior art and form no part of the present invention.
The prior art shift control systems of the preselectable type for change gear transmissions are highly advantageous as they permit the operator to preselect a gear change which gear change will be executed upon subsequent achievement of certain operating conditions, such as a break in the torque allowing disengagement of the previously engaged gear and achievement of substantially synchronous rotation of the jaw clutch members associated with the selected gear. However, under certain situations, the prior art shift control systems of the preselection type have been undesirable in situations wherein a gear change is preselected and the required operating conditions are not achieved for a relatively long period of time allowing the operator to forget that such a preselection has occurred which may result in an unexpected gear change being executed upon later achieving of such required operating conditions. By way of example, if an operator preselects a gear change and road conditions change to the extent that he does not break the torque in the system by depressing the clutch or releasing the throttle, execution of the preselected gear change at a later time may be unexpected to the operator. This situation is more likely to occur when the preselected gear change is an auxiliary section gear change achieved by mere movement of a selector lever on the shift knob rather than if the preselected gear change is a main section gear change requiring movement of the gear shift lever itself.